The invention relates to a process for drilling holes in the earth's crust with a drill bit, with use of cold drilling liquids to be applied in any process for recovering energy.
It is known for holes to be drilled in the crust of the earth with use of cold drilling liquids so that the wall of the drill hole is frozen. Certain clay formations, which are unstable if non-cooled drilling liquids are used and affect the drilling process detrimentally, retain their coherence and stay in place. Also, if these cold drilling liquids are used cores can be extracted from unconsolidated rocks with a higher yield since these cores freeze and do not disintegrate easily during the drilling process.
It will be clear that the temperature of the circulating medium should be lower than the freezing point of the liquid contents of the formation to be frozen and equal to or higher than the freezing point of the circulating medium itself. For drilling liquid use may be made of various liquids, such as salt water, a salt water--mud flush, oil, e.g. Diesel oil, or a water/oil emulsion. Chilling of the drilling liquid may take place according to known methods; solid carbon dioxide is often used for this purpose. The drill pipes may have been provided with insulation material in order to effect transportation of as much cold as possible to the bottom of the drill hole.
A problem not connected with this is the drilling of holes having a small diameter (so-called `slim holes`0 through formations comprising soft or plastic clays, or soft shales (slates). Under these conditions so-called insert drill bits, diamond drill bits or other drill bits suitable for drilling hard formations can make little progress as a result of the so-called `balling up` effect. In order to keep the drill bit as long as possible at the bottom of the drill hole it is desirable, just because of the slight wear, to use one of these types of drill bit. Generally, the diamond drill bit will be preferred because of the absence of bearings.